docs: def unschedule(ident):
      '''Unschedule an harvesting on a source'''
      source = get_source(ident)
      if not source.periodic_task:
          msg = 'Harvesting on source {0} is ot scheduled'.format(source.name)
          raise ValueError(msg)
  
      source.periodic_task.delete()
      signals.harvest_source_unscheduled.send(source)
      return source